Mitchell Point was the site of an iconic tunnel with its five arched windows overlooking the Columbia River. Constructed in 1915, the tunnel was closed in 1953 because it could no longer accommodate high traffic volumes and car sizes. It was destroyed in 1966 to widen the water-level highway, now Interstate 84. The historic tunnel was 390-feet long. Mitchell Point is a formidable mountain that requires a new crossing to connect the western segments of State Trail to Hood River and The Dalles.
The total length of the tunnel is approximately 647 feet. The tunnel was excavated in a horseshoe shape, with a finished inside crown radius of 8.5 feet and vertical sidewalls 17 feet wide that provided the required 12-foot-wide trail surface with 2-foot minimum shoulders. Inside the tunnel, beyond each portal structure, the tunnel transitioned to a full circumference shotcrete lining for the full length. The tunnel was constructed with five viewing adits projecting out to the north with varying lengths. The section geometry for each adit was a modified horseshoe shape slightly smaller than the main trail tunnel.
LRL means and method for the construction of the main line and five adits was performed by drill and blast method. Approximately 8,000 cubic yards of rock was generated during the construction of the tunnel and adits. A total of 1,500 liner feet of #11 spiling bars were installed around each opening of the tunnel and adits. Approximately 12,000 liner feet of solid rock dowels were installed in the crown and sidewalls of the tunnel and adits. Fourteen steel sets consisting of W8x40 members were installed along the western portion of the main tunnel. LRL installed approximately 5,000 square yards of structural shotcrete and 3,000 square yards of colored shotcrete within the main tunnel and adits.
Columbia River, OR